1. Field of the Invention
The present invention relates to an apparatus for and a method of forming an electrode for a lithium secondary cell, and more specifically, it relates to an apparatus for and a method of forming an electrode for a lithium secondary cell for forming an active material layer on the surface of a collector.
2. Description of the Prior Art
In a lithium secondary cell actively subjected to research and development in recent years, the cell characteristics such as a charge/discharge voltage, an operating cycle lifetime and shelf stability remarkably depend on an electrode employed therein. Therefore, an active material employed for the electrode is improved for upgrading and improving the cell characteristics.
For example, a lithium secondary cell employing aluminum, Si or tin electrochemically alloyed with lithium in charging as a negative electrode active material is proposed in general, as reported in Solid State Ionics, 113-115, p 57 (1998) or the like. Among aluminum, Si and tin, Si having large theoretical capacity is particularly promising as a negative electrode active material for a cell exhibiting high capacity. Therefore, various lithium secondary cells having negative electrode active materials of Si are proposed.
In order to form the aforementioned negative electrode active material consisting of Si, Si is generally deposited on a collector from a sputtering source containing a target consisting of Si, thereby forming a negative electrode active material layer of Si on the collector.
In the aforementioned negative electrode active material layer consisting of only Si formed on the collector, however, the volume of Si alloyed with lithium thereby storing lithium is remarkably expanded and shrinked following charge/discharge reaction. In charging/discharging, therefore, Si is so powdered (pulverized) that the negative electrode active material layer consisting of Si is disadvantageously separated from the collector to deteriorate the operating cycle characteristics.
In general, therefore, a technique of preventing pulverization of Si by adding a foreign element to Si is developed. Pulverization of a thin film of Si can conceivably be suppressed by introducing the foreign element into the Si thin film and changing the mechanical/physical properties of the thin film. Therefore, it is important that the thin film homogeneously contains the foreign element. In the aforementioned conventional sputtering, however, it is difficult to form such a negative electrode active material layer containing Si and the foreign element added thereto on the collector. In other words, it is difficult to form the negative electrode active material containing Si and the foreign element added thereto in the conventional sputtering employing only a single target (single sputtering source) consisting of Si.
An object of the present invention is to provide an apparatus for forming an electrode for a lithium secondary cell capable of readily forming an active material layer constituted by at least two elements and controlling the composition of the active material layer.
Another object of the present invention is to form the active material layer constituted by at least two elements to have homogeneous concentration distribution in the aforementioned apparatus for forming an electrode for a lithium secondary cell.
Still another object of the present invention is to provide a method of forming an electrode for a lithium secondary cell capable of readily forming an active material layer constituted by at least two elements and controlling the composition of the active material layer.
A further object of the present invention is to form the active material layer constituted by at least two elements to have homogeneous concentration distribution in the aforementioned method of forming an electrode for a lithium secondary cell.
In order to attain the aforementioned objects, an apparatus for forming an electrode for a lithium secondary cell according to a first aspect of the present invention, employed for forming an active material layer on the surface of a collector, comprises a first sputtering source for sputtering a first material forming the active material layer onto the surface of the collector and a second sputtering source for sputtering a second material forming the active material layer onto the surface of the collector. A plasma region of the first sputtering source and a plasma region of the second sputtering source are arranged to overlap with each other.
The apparatus for forming an electrode for a lithium secondary cell according to the first aspect is provided with the first sputtering source and the second sputtering source for sputtering the first material and the second material forming the active material layer respectively onto the surface of the collector while arranging the plasma region of the first sputtering source and the plasma region of the second sputter region to overlap with each other as hereinabove described, whereby an active material layer constituted by at least two elements can be readily formed with excellent reproducibility. When power applied to the first sputtering source and that applied to the second sputtering source are controlled independently of each other in this case, the composition of the active material layer constituted by at least two elements can be readily controlled.
In this case, a resulting thin film preferably forms a solid solution of the first material (e.g., Si) and the second material (e.g., Co). In the above example, Co is preferably contained in Si not as an intermetallic compound of Si and Co but in the form of a solid solution. The term xe2x80x9cintermetallic compoundxe2x80x9d stands for a compound formed by combining metals at a specific ratio to have a specific crystal structure. The solid solution is preferably in a non-equilibrium state. Only Ge is known as an element forming a solid solution with Si in an equilibrium state, and the solid solution of Si and a foreign element according to the aforementioned aspect exists only in a non-equilibrium state. In this point of view, formed Si is preferably in an amorphous or microcrystalline state, and sputtering which is a thermally non-equilibrium process, CVD, vacuum deposition or the like is preferably employed for forming Si.
The aforementioned apparatus for forming an electrode for a lithium secondary cell according to the first aspect preferably further comprises a collector holding part capable of holding the collector and relatively moving the collector with respect to the first sputtering source and the second sputtering source. The collector holding part capable of relatively moving the collector with respect to the first sputtering source and the second sputtering source is so provided that the composition ratio (concentration distribution) of the active material layer can be homogenized by moving the collector holding part when forming the active material layer constituted by at least two elements on the surface of the collector. Thus, the active material layer constituted by at least two elements can be readily formed with excellent reproducibility and homogeneous composition.
In this case, the collector holding part preferably includes means cooling the collector. According to this structure, the collector can be cooled when the active material layer constituted by at least two elements is formed thereon. Thus, the collector component can be prevented from excessively diffusing into the active material layer when the collector is at a high temperature. Further, strain (internal stress) resulting from difference in thermal expansion can also be prevented. Consequently, it is possible to prevent deterioration of charge/discharge characteristics resulting from excessive diffusion of the collector component in the active material layer or internal stress of the temperature leading to separation of the active material layer.
In the aforementioned structure comprising the collector holding part, the collector holding part preferably has a substantially cylindrical form, and the first sputtering source and the second sputtering source are preferably arranged to enclose the collector holding part. When the collector holding part is formed to have a substantially cylindrical form while arranging the first and second sputtering sources to enclose the collector holding part, the active material layer constituted by at least two elements can be readily formed on the collector with excellent reproducibility.
The aforementioned apparatus for forming an electrode for a lithium secondary cell according to the first aspect preferably further comprises an anti-adhesion member set on a region other than the area where the plasma region of the first sputtering source and the plasma region of the second sputter region overlap with each other. According to this structure, the active material layer can be formed on the collector only on the area where the plasma regions of the first and second sputtering sources overlap with each other.
In the aforementioned apparatus for forming an electrode for a lithium secondary cell according to the first aspect, the first sputtering source preferably includes a first target consisting of the first material, and the first material forming the first target preferably contains at least Si. When the first target of the first sputtering source contains at least Si and a target consisting of a foreign element is employed as the second sputtering source, the active material layer can be readily formed by adding the foreign element to Si. In this case, the first material forming the first target preferably contains at least one element selected from a group consisting of Cu, Co, Fe, Zn, Zr, Mn, Ni and Ag in addition to Si. When the first target of the first sputtering source is prepared from the material containing Si and the foreign element and a target consisting of the same foreign element as the above is employed as the second sputtering source, the composition ratio of Si and the foreign element can be more readily controlled when forming the active material layer by adding the foreign element to Si.
In the aforementioned apparatus for forming an electrode for a lithium secondary cell, the second sputtering source preferably includes a second target consisting of the second material, and the second material forming the second target preferably contains a metallic element. When the second material of the second sputtering source contains a metallic element and the target containing Si is employed as the first sputtering source, the active material layer can be readily formed by adding the metallic element to Si. In this case, the metallic element preferably includes at least one element selected from a group consisting of Cu, Co, Fe, Zn, Zr, Mn, Ni and Ag. When such a metallic element is employed as the component of the second target of the second sputtering source and Si is employed as the first sputtering source, the active material layer can be readily formed by adding the aforementioned metallic element to Si. In this case, pulverization of Si can be readily prevented by adding the aforementioned metallic element to Si.
In the aforementioned apparatus for forming an electrode for a lithium secondary cell according to the first aspect, the first sputtering source preferably includes a first target consisting of the first material, the first material forming the first target is preferably Si, the second sputtering source preferably includes a second target consisting of the second material, and the second material forming the second target is preferably Co. According to this structure, the active material layer consisting of Si and Co can be readily formed.
In the aforementioned apparatus for forming an electrode for a lithium secondary cell according to the first aspect, power is preferably separately supplied to the first sputtering source and the second sputtering source respectively. According to this structure, power applied to the first sputtering source and that applied to the second sputtering source can be controlled independently of each other, whereby the composition of the active material layer consisting of the first and second materials can be readily controlled. When the power is separately applied, the frequency of a power source employed for the first sputtering source may be rendered different from the frequency of a power source employed for the second sputtering source. According to this structure, the two power sources can be prevented from interfering with each other. Thus, the two power sources can be readily controlled independently of each other. When the power is separately applied, a high-frequency power source may be employed for the first sputtering source, and either a DC power source or a pulse power source may be employed for the second sputtering source. According to this structure, the two power sources can be readily prevented from interfering with each other.
A method of forming an electrode for a lithium secondary cell according to a second aspect of the present invention, employed for forming an active material layer on the surface of a collector, comprises steps of arranging a first sputtering source for sputtering a first material forming the active material layer onto the surface of the collector and a second sputtering source for sputtering a second material forming the active material layer onto the surface of the collector so that plasma regions of the first sputtering source and the second sputtering source overlap with each other and sputtering the first material onto the surface of the collector by the first sputtering source while sputtering the second material onto the surface of the collector by the second sputtering source.
In the method of forming an electrode for a lithium secondary cell according to the second aspect, the first sputtering source and the second sputtering source so arranged that the plasma region thereof overlaps with that of the first sputtering source are employed for sputtering the first and second materials onto the surface of the collector as described above, whereby an active material layer constituted by at least two elements can be readily formed with excellent reproducibility. When power applied to the first sputtering source and that applied to the second sputtering source are controlled independently of each other in this case, the composition of the active material layer constituted by at least two elements can be readily controlled with excellent reproducibility.
In the aforementioned method of forming an electrode for a lithium secondary cell according to the second aspect, the step of sputtering the first material and the second material forming the active material layer preferably includes a step of sputtering the first material and the second material onto the surface of the collector from the first sputtering source and the second sputtering source while relatively moving the collector with respect to the first sputtering source and the second sputtering source. When the collector is relatively moved with respect to the first and second sputtering sources for sputtering the first and second materials onto the surface of the collector from the first and second sputtering sources, the composition ratio (concentration distribution) of the active material layer containing the first and second materials can be homogenized. Thus, the active material layer constituted by at least two elements can be readily formed in homogeneous composition.
In the aforementioned method of forming an electrode for a lithium secondary cell, the step of sputtering the materials forming the active material layer preferably includes a step of sputtering the first material and the second material onto the surface of the collector from the first sputtering source and the second sputtering source respectively while cooling the collector. When the first and second materials are sputtered onto the surface of the collector from the first and second sputtering sources while cooling the collector, the collector component can be prevented from excessively diffusing into the active material layer when the collector is at a high temperature. When the collector component properly diffuses into the active material layer, pulverization can be suppressed and the quantity of expansion/shrinkage of a thin film can be reduced, similarly to the aforementioned case of adding the foreign element to Si. Thus, stress applied from the thin film to the collector can be reduced in charging/discharging, thereby preventing separation of the thin film from the collector. When the collector component largely diffuses into the active material layer, however, the quantity of Si contributing to charging/discharging is reduced to lower the cell capacity. It is important to prevent excessive temperature increase, in order to prevent internal stress resulting from difference in expansion between the collector and a substrate and conversion from a solid solution to an intermetallic compound.
In the aforementioned method of forming an electrode for a lithium secondary cell, the step of sputtering the first material and the second material forming the active material layer preferably includes a step of separately supplying power to the first sputtering source and the second sputtering source respectively thereby controlling the composition of the active material layer consisting of the first material and the second material sputtered onto the surface of the collector. When the power is separately supplied to the first and second sputtering sources, the power applied to the first sputtering source and that applied to the second sputtering source can be controlled independently of each other, whereby the composition of the active material layer consisting of the first and second materials can be readily controlled. When the power is separately applied, the frequency of a power source employed for the first sputtering source may be rendered different from the frequency of a power source employed for the second sputtering source. According to this structure, the two power sources can be prevented from interfering with each other. Thus, the two power sources can be readily controlled independently of each other. When the power is separately applied, a high-frequency power source may be employed for the first sputtering source, and either a DC power source or a pulse power source may be employed for the second sputtering source. According to this structure, the two power sources can be readily prevented from interfering with each other.
The aforementioned method of forming an electrode for a lithium secondary cell according to the second aspect preferably further comprises a step of roughening the surface of the collector in advance of the sputtering step. According to this structure, adhesion between the active material layer and the collector can be improved.
In the aforementioned method of forming an electrode for a lithium secondary cell according to the second aspect, the first sputtering source preferably includes a first target consisting of the first material, and the first material forming the first target preferably contains at least Si. According to this structure, the active material layer can be readily formed by adding a foreign element to Si when employing a target of the foreign element as the second sputtering source. In this case, the first material forming the first target preferably contains at least one element selected from a group consisting of Cu, Co, Fe, Zn, Zr, Mn, Ni and Ag in addition to Si. When a target consisting of a material containing Si and a foreign element is employed as the first target of the first sputtering source and a target consisting of the same foreign element as the above is employed as the second sputtering source, the composition ratio between Si and the foreign element can be more readily controlled when forming the active material layer by adding the foreign element to Si.
In the aforementioned method of forming an electrode for a lithium secondary cell according to the second aspect, the second sputtering source preferably includes a second target consisting of the second material, and the second material forming the second target preferably contains a metallic element. When the second material of the second sputtering source contains the metallic element and the target containing Si is employed as the first sputtering source, the active material layer can be readily formed by adding the metallic element to Si. In this case, the metallic element preferably includes at least one element selected from a group consisting of Cu, Co, Fe, Zn, Zr, Mn, Ni and Ag. When such a metallic element is employed as the component of the second target of the second sputtering source and Si is employed as the first sputtering source, the active material layer can be readily formed by adding the aforementioned metallic element to Si. In this case, pulverization of Si can be effectively prevented by adding the aforementioned metallic element to Si.
In the aforementioned method of forming an electrode for a lithium secondary cell according to the second aspect, the first sputtering source preferably includes a first target consisting of the first material, the first material forming the first target is preferably Si, the second sputtering source preferably includes a second target consisting of the second material, and the second material forming the second target is preferably Co. According to this structure, the active material layer consisting of Si and Co can be readily formed.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.